This invention relates to a thermal flow sensor and a heat-sensitive resistor therefor.
Hitherto, a method of detecting the flow of a fluid from the equilibrium state of a bridge circuit including a heat-sensitive resistor disposed in the fluid flow has been applied to thermal flow sensors such as that disclosed in Japanese Utility Model Laid-Open No. 61-108930. A description will be given, with reference to some of the drawings, of a conventional air flow sensor in which a head-sensitive resistor is employed as a heating resistor which comprises a ceramic substrate and a platinum thin-film resistor formed on the substrate.
FIG. 11 schematically shows the arrangement of the conventional thermal flow sensor in which a heat-sensitive resistor is provided. As shown in the figure, a sensor tube 2 is provided at a predetermined position within a housing 1 defining the main passage of a fluid. A heat-sensitive electrical resistor 3 and an air temperature sensor 4 are disposed on the sensor tube 2. Each of the group consisting of the heat-sensitive resistor 3 and a resistor R2 and the group consisting of the air temperature sensor 4 and another resistor R1 is connected in series and these elements form a bridge circuit.
The heat-sensitive electrical resistor 3 has a structure such as that shown in FIGS. 12 and 13 which will be described in detail later. The thermal flow sensor shown in FIG. 11 also includes a control circuit in which the junction 7 between the heat-sensitive resistor 3 and the air temperature sensor 4, partially forming the bridge circuit, is connected to the emitter of a transistor 102, Also in this circuit, the junction 5 between the heat-sensitive resistor 3 and the resistor R2, and the junction 6 between the air-temperature sensor 4 and the resistor R1 are connected to the input terminals of a differential amplifier 101. The output of the differential amplifier 101 is applied to the base of the transistor 102. The collector of the transistor 102 is connected to the positive electrode of a d.c. power source 103, the negative electrode of the power source 103 being grounded.
FIGS. 12 and 13 are a front view and a side view, respectively, of the heat-sensitive electrical resistor 3 shown in FIG. 11. In these figures, the heat-sensitive electrical resistor 3 comprises an electrically insulating elongated base plate 31 supported at one end thereof by the detection tube 2. The base plate 31 is made of alumina. The base plate 31 has attached thereon a heat-sensitive resistor element 32 including a sepertine portion 33 made of a heat sensitive resistor material such as platinum having a resistance which changes with temperature at a positive temperature coefficient. The heat-sensitive resistor element 32 is provided with a pair of lead lines 34 and coated with a protective coating 35.
The operation of the thermal flow sensor having the above-described construction is already known, so that the operation will not be described in detail, and it will be only briefly outlined, When the voltage at the junction 6 and that at the junction 5 have become equal to each other, the bridge circuit achieves its equilibrium state. When a fluid such as air flows through the detection housing 1, the bridge circuit is maintained at its equilibrium state by adjusting the current supplied to the bridge circuit so that the mean temperature of the heat-sensitive resistor element 32 of the heat-sensitive resistor 3 is at a temperature higher than the temperature of the fluid by a predetermined amount. When the fluid speed increases in this state, the heat-sensitive resistor element 32 is cooled and its resistance increases, whereby the equilibrium state of the bridge circuit is destroyed. At this time, the control circuit causes the current supplied to the bridge circuit to increase to additionally heat the heat-sensitive resistor element 32 so that its mean temperature is returned to its initial value, whereby the equilibria state of the bridge circuit is recovered and the flow speed of the fluid can be obtained from the supplied current amount.
However, in the conventional thermal flow sensor, the heat generated at the heat-sensitive resistor element 32 is dissipated not only into the fluid contacting the resistor element 32 but also into the support structure such as the detection tube 2 through the supported end. Therefore, the temperature profile of the heat-resistive electric resistor 3 is as shown in FIG. 14, from which it is seen that the temperature of the resistor 3 substantially linearly changes according to the height position h0 through h5 from the highest temperature Tmax at its free end to the lowest temperature at its supported end. Since the heat-sensitive resistor element 32 is made of a material such as platinum having a positive temperature coefficient of resistance, the higher-temperature portion of the resistor element 32 has a higher resistance and is further heated and the lower-temperature portion of the resistor element 32 has a lower resistance which provides a lower temperature, so that the temperature difference is increased between the highest temperature T.sub.max of the resistor element 32 and the mean temperature T.sub.mean. This tendency is further increased when the base plate 31 is made of a good thermally conductive material such as alumina.
On the other hand, since the mean temperature of the heat-sensitive resistor element 32 is controlled at a constant temperature by the control circuit, the local highest temperature T.sub.max of the heat-sensitive resistor element 32 is increased when the difference between the highest temperature T.sub.max and the mean temperature T.sub.mean is large. In the thermal flow sensor, the heat of the heat-sensitive resistor 3 is also dissipated by heat radiation. Since the amount of heat radiation proportionally increases by the fourth power of the absolute temperature of matter, the amount of heat radiation can be considered dependent upon the local highest temperature T.sub.max. Accordingly, with the above-arrangement in which the highest temperature T.sub.max is high, the measurement error is large due to the heat radiation. Also, since the proportion of the heat dissipated by conduction through the supported end from the heat-sensitive resistor 3 to the total generated heat in the heat-sensitive resistor 3 changes in accordance with the flow rate of the fluid, the temperature profile or the distribution on the resistor 3 varies in accordance with the fluid speed. Therefore, when the fluid speed changes abruptly, the operation of the control circuit is transitional and no normal correct output can be obtained until the temperature profile of the heat-sensitive resistor 3 reaches a stable temperature profile corresponding to the flow rate at that time.
Thus, in the conventional heat-sensitive resistor as above described, the highest temperature T.sub.max is relatively high as compared to the mean temperature T.sub.mean of the heat-sensitive resistor element 32, so that the measurement error due to the effect of the heat radiation from the heat-sensitive resistor 3 is large and the abrupt change in flow rate cannot be precisely followed.